Little help needed to understand Dimmer for DC lamp?

[Byte_Butcher]

Hmmm. Well, it seems like there's an awful lot of < $2 N-channel MOSFETS that will easily switch 10 or more amps with gate voltages well under 5 volts. You've got P-channel MOSFETS cheaper than that? Plus a NPN?

I use a 16F684 and N-channel MOSFET to PWM control power to a 50 watt 12 volt heater element @ 100Hz with no issues.
I suppose the question is how much current is Megahertz talking about? Small 25-50W halogens, or 250 watt driving lights?

As long as the frequency isn't too high there shouldn't be any trouble driving an N-channel MOSFET directly from a PIC. For driving a halogen lamp there's probably no need for more than about 100 Hz.

If your PIC is running at 3 volts or less, then the choice of logic level high current FETs does diminish greatly and a driver circuit may be necessary.

I suppose it might be worth mentioning that dimming a halogen bulb reduces the operating temperature and disrupts the halogen cycle which can lead to premature envelope blackening and early death. For cheap garden variety bulbs, you may not care. But if you're playing with expensive photography bulbs or something like that, then it might be worth mentioning...

Steve

[Megahertz]

Easily available MOSFET I found out is IRF540 -
This is link is to the datasheet:
http://www.irf.com/product-info/data...ta/irf540n.pdf

Since I am not familier with most of the terms in the datasheet, can you please advise if I can use this Mosfet and If I can use the same circuit?

Also, I need to control lamps upto 100W, which I believe can be done using Heatsink on this Mosfet. Thanks for the help.

[aratti]

can you please advise if I can use this Mosfet and If I can use the same circuit?

No! IRF540 is a N channel mosfet the schematic uses a P channel mosfet.

Al.

[Megahertz]

No! IRF540 is a N channel mosfet the schematic uses a P channel mosfet.

Al.

Thanks Al, provided the circuit is changed, will this mosfet be able to do the Job?
I normally keep the components as advised, but IRF9540 is not normally available where I am.

[aratti]

....provided the circuit is changed, will this mosfet be able to do the Job?

Yes, it will work.

Al.

[Charles Linquis]

Use an N-channel FET if you are going to switch the GND side of the circuit. Use a P-channel FET if you are going to switch the positive (high) side. If you switch the high side, you will need a level shifter like Aratti has shown. If you switch the low side, you don't need the level shifter and can drive the FET directly.

Switching the high side is necessary if one side of your lamp must be grounded. Since I doubt that is the case, then I would recommend using an N-channel logic-level FET with the source connected to GND, the drain connected to the lamp (and the other lamp terminal connected to 12V), and the GATE connected to the PIC. Use a 10K pull DOWN resistor between the gate and GND, so that the lamp is off until your program gets started.

At the low frequencies you are running, most of your losses will be resistive. Look at the RDSON with the gate drive you will provide (probably around 4V). If it is 50 milli-ohms, then at 10A the FET will have a (.050*10) = .5V drop. .5V X 10A = 5 Watts. The lower RDSON, the lower the amount of heat produced. My guess is that you will produce maximum heat at around 80-90% of full brightness.

[Megahertz]

I found one mosfet P30NF10
datasheet: http://www.st.com/stonline/products/...re/ds/7817.pdf

The setup worked upto some extent but with few concerns.
firstly when system was fired up, the lamp started glowing but very DIM. I increased the resistance with 1.5K more with 4.7K and this problem got sorted. Now the lamp stays OFF when system is powered up

The problem is that the lamp is not glowing to its full potential. Any ideas please as what changes I can try in the hardware to make this work, i.e. make the lamp off at value 0 & glow at full potential at 255.
I am using 20KHz as frequency with HPWM command.
Thanks

[Megahertz]

I use a 16F684 and N-channel MOSFET to PWM control power to a 50 watt 12 volt heater element @ 100Hz with no issues.
I suppose the question is how much current is Megahertz talking about? Small 25-50W halogens, or 250 watt driving lights?

As long as the frequency isn't too high there shouldn't be any trouble driving an N-channel MOSFET directly from a PIC. For driving a halogen lamp there's probably no need for more than about 100 Hz.

If your PIC is running at 3 volts or less, then the choice of logic level high current FETs does diminish greatly and a driver circuit may be necessary.

Steve

Hi Steve, this is the only mosfet I could find locally, I am in India currently and I have to develop this using easily available components. Though I will try once again to find P-Channel, but I would like to request you if you can upload the circuit here for N-Channel mosfet you use for 12V - 50W heater.

I need it for 50W @ 12V halogen.
My PIC is running on 5V.
Widely available mosfet is IRF540 (N-Channel)

All help is much appreciated. Thanks